An extrusion film sheet forming apparatus, a lamination pinching apparatus, and the likes have been used as an apparatus for forming film-like sheets of synthetic resins. The extrusion film sheet forming apparatus is used for rolling a molten synthetic resin material into a film-like sheet and a lamination pinching apparatus is used for applying pressure to laminated film-like sheets to cause pressure-bonding of the sheets.
A metal or rubber roll is often used for such a rolling or pressure-bonding process. For example, the extrusion film sheet forming apparatus has a pressure-bonding roll, a cast roll provided in opposition to the pressure-bonding roll, and a drive unit to rotate the cast roll. With such a configuration, a thin film-like sheet made of a synthetic resin material is formed by feeding the synthetic resin material in a molten state between the pressure-bonding roll and the cast roll thus causing the material to be pressured between the two rolls. The cooling of the two rolls is accomplished by feeding a cooling medium inside either of the rolls.
When the metallic roll is used as a pressure-bonding roll used in such an apparatus, it may cause the thickness of the film-like sheet made of a resin material, which is the material being formed, thinner, as the roll itself does not provide flexibility in the radial direction, and it may be difficult to execute the rolling or pressure-bonding process properly if there is any thickness inaccuracy along the width direction of the roll.
On the other hand, when the rubber roll is used as a pressure-bonding roll, a pressure-bonding or rolling does not cause any unevenness of the thickness in the film-like sheet thus produced thanks to the flexibility of the roll in the radial direction. However, the rubber roll cannot be used if the temperature of the material to be formed is high. For example, if the temperature of the rubber roll rises during the pressure-bonding or rolling, the material to be formed may stick or get wrapped around the roll, making the forming impossible. In particular, when the roll surface is made of rubber, it is practically impossible to produce mirror finish like a metallic surface or transfer sculptured patterns from the roll.
In order to cope with such a problem, an extrusion sheet forming roll equipped with a rubber layer on the outer periphery of a shaft core part, a tubular member provided on the outer periphery of said rubber layer, and a metallic film to cover the surface of said tubular member, is disclosed in the Publication of Japanese Patent No. 3795999.
This extrusion sheet forming roll is used for the extrusion forming of a plastic sheet made of thermoplastic resin having a flat and smooth surface at least on one side. Such a roll is made by forming a seamless tubular metallic member from a material mainly consisting of Nickel by the electrocasting method, covering the outside of the tubular metallic member with a film primarily consisting of Chromium, and inserting a rubber layer to cover the shaft core part into the tubular metallic member by the shrink-fitting method so that it would be affixed solidly due to the friction force when it cools down to the normal temperature.
This extrusion sheet forming roll, however, has a problem that it cannot be cooled quickly when it is intended to cool the high temperature of the tubular metallic member with the cooling medium from the shaft core part side, as the shaft core part is covered by the heat insulating rubber layer. Consequently, the material to be formed may get stuck on the extrusion sheet forming roll during the forming of the film sheet, making it difficult to form the film sheet. It also makes the transfer of the pattern sculptured on the roll to the sheet difficult.
Furthermore, the high temperature of the material to be formed may cause the tubular metallic member to expand thermally, thus causing the tubular member to expand circumferentially and axially, allowing the tubular member to offset axially or rotate in the circumferential direction. As a result, distortion, or even damage, may develop in the tubular metallic member.